Nitrones from Addition of Benzyl and Allyl Grignard Reagents to Alkyl Nitro Compounds: Chemo-, Regio-, and Stereoselectivity of the Reaction

Article abstract of DOI:10.1021/jo00048a012

Reaction of allyl and benzyl Grignard reagents with functionalized nitroalkanes afford nitrones in good yield.This process shows considerable chemoselectivity; carbonyl groups and other highly reactive electrophilic functions are unaffected by the reaction conditions (THF, -70 deg C).A mixture of regioisomers 4 and 5 is usually obtained, and the product distribution depends on the nature of the alkyl framework.An intermediate 3 is postulated, and the isomeric pair of nitrones arises, very likely through a selective syn elimination. α-Substituted alkyl chains give mostly conjugated products 4 while unbranched chains afford predominantly the nonconjugated nitrones 5.The 4/5 ratio can be strongly modified by using a proton source of suitable strength; trichloroacetic acid produces 4 exclusively in the reaction of nitroethane with benzyl Grignard, while 2,6-dimethylphenol affords completely the nonconjugated nitrone 5.The stereochemistry of the double bond is affected by the nature of the reagent used.Benzyl Grignard gives only Z nitrones 4 and 5; 2-butenylmagnesium chloride gives nonconjugated Z nitrones and a predominance of E isomer in the conjugated nitrone 5.